The method and apparatus of the present invention relate generally to water softening sequestration and, more particularly, to automatic end-point detection in sodium tripolyphosphate sequestration of hardness elements of a wash water solution irrespective of variations of concentration levels of hardness elements in the wash water solution or in the soils being removed by the wash water solution.
It is well known that the hardness elements in a wash water solution affect detrimentally the level of satisfactory detergent cleaning of objects being washed. The hardness of water is principally due to its calcium (Ca) and magnesium (Mg) content. Hereinafter, such total water hardness is expressed in terms of an equivalent amount of calcium carbonate (CaCO.sub.3).
When detergent is added to the wash water solution, the Ca and Mg ions in the wash water solution tend to react with various ions present in the detergent. These reactions may result in the precipitation of essentially insoluble Ca and Mg compounds, which results in loss of detergent values. Thus, the hardness elements in the wash water solution must be dealt with in some fashion in order to obtain the desired cleaning level using a reasonable detergent level.
Sequestration is the preferred embodiment for achieving in situ softening of hard water in washing processes. In the sequestration method, a metal complexing agent is added to the wash water solution, and the agent chelates the hardness ions to form soluble chelate salts. These chelate salts are stable and prevent the formation of insoluble compounds that might otherwise form. It is well known that the ideal concentration level of the sequestration agent in the wash water solution is the concentration level which chelates all of the hardness values and results in a small amount of excess sequestering agent.
In many applications, for example, in the home washer environment, it is impossible to select a fixed amount of sequestering agent to be added to the wash water solution in order to obtain the desired ideal sequestration. This inability to fix the needed amount of sequestering agent, even when the volume of the wash water solution is fixed, is due to variations in the concentration levels of hardness elements in the wash water solution and in the soils being removed by the wash water solution. Specifically, the amount of Ca and Mg present in surface water supplies varies seasonably in response to rainfall variations, snow melt, etc. The variation of these constituents in underground water supplies also varies for many reasons. Further, the amount of Ca and Mg present in the wash water solution is variable and increases if the soil load being removed by the wash water solution contains these elements. Many normally occurring soils do contain these hardness elements, which results in an increase in the actual concentration of both Ca and Mg from their base-line levels in the wash water supply. These added amounts of Ca and Mg also vary unpredictably with the nature and amount of soils present.
In an attempt to predict the amount of sodium tripolyphosphate sequestering agent needed to soften completely the wash water solution in a wash water apparatus, for example, a home dishwasher unit, appliance manufacturers have generally resorted to suggesting that the user start at some modest level of sequestering agent usage and then adjust the level according to the average need as perceived by the user. This trial-and-error approach obviously is unlikely to assure the use of the optimally efficient amount of sodium tripolyphosphate softener for any given wash load.
A further problem with conventional methods for softening water using sodium tripolyphosphate is that the sodium tripolyphosphate often is a component of the detergent composition being added to the wash water solution. In the case of home dishwashers, for example, sodium tripolyphosphate composes between 28-50% of the chemical ingredients in most commercially available dishwasher detergent compositions. Often, the amount that is required to be added to the wash water solution in order to achieve the desired softening level far exceeds the detergent requirements. In other words, the detergent requirements for surface activity, alkaline builders, corrosion inhibitors, suds suppressors, and chlorine carriers are not increased linearly with increased water hardness elements requiring the sodium tripolyphosphate component of the detergent composition. Thus, detergent is wasted, which is detrimental both from an economic and ecological viewpoint.
Lastly, it also would be highly beneficial both from an economic and an ecological viewpoint to be able accurately to predict on an in situ basis the amount of sodium tripolyphosphate necessary to achieve the desired softening of the wash water solution.